The present invention relates to rock reinforcing structure, and more specifically, to novel elements and methods of manufacture thereof for use in resin anchored, tensioned roof support systems.
In mine work, such as coal mining, or in underground formations such as tunnels or excavations, it is often necessary to reinforce or support the roof and/or walls of the excavation to prevent rock falls or cave-ins. The most common means presently in use for effecting such support include elongated bars or bolts which are inserted in blind drill holes and anchored therein to hold a metal support or bearing plate in tight engagement with the roof or wall surface. Anchoring means within the drill hole normally comprise a mechanical expansion anchor, a hardenable resin or other grouting, or both.
The use of polyester resins in underground formations was disclosed at least as early as the June 4, 1963 U.S. Pat. No. 3,091,935 of Brown and Pritchard entitled "Rock Treatment." It has been found expedient to provide the resin grouting materials in two components each of which remains in a semi-liquid or thixotropic phase until mixed with the other, whereupon curing and hardening progresses to the stage that the steel bolt or bar will fail before the resin bond. The two components, a polyester resin and a catalyst, curing or hardening agent, are commonly provided in a single cartridge wherein they are initially separated in individual compartments such as disclosed, for example, in U.S. Pat. No. 3,324,663 of McLean. Upon insertion of the resin cartridge and elongated bar into the drill hole, the end of the bar contacting the cartridge is used to rupture the two-compartment cartridge and mix the components so that the curing and hardening necessary to retain the bar in the hole may take place.
In addition to anchoring one end of the bar in the drill hole it is also desirable that the bar be placed in tension in order to reinforce the rock formation surrounding the excavation. To this end, a number of systems have been proposed wherein the bolt or bar is threadedly engaged with another member, with some means being provided to limit threaded advance as the resin cartridge is broken and the components mixed, and excess torque applied after the resin has hardened serving to break or deform some portion of the structure to allow further threaded movement and tensioning of the bar. The earliest known example of such anchor systems is Hill U.S. Pat. No. 3,877,235 wherein a relatively thin plate or diaphragm covering one end of a threaded nut is broken by applying excess torque after hardening of the resin grouting. In Koval U.S. Pat. No. 4,051,683 a shear pin extending across an internally threaded coupling member is broken by application of excess torque. In Vass, et al U.S. Pat. No. 4,122,681 initial threaded advance is limited by internally deformed threads, and further threaded advance to tension the bar or bolt is effected by application of excess torque after the resin hardens.
Of the considerable number of proposed systems for tensioning resin-anchored bars or bolts by application of excess torque, many involve structural alteration of, or the insertion of frangible elements directly into, threaded elements of the system. This presents the possibility of failure due to seizing, fouling or stripping of the threads. Also, many of the previously proposed systems require costly structural additions and/or fabrication operations which diminish the economic effectiveness of the system.
It is a principal object of the present invention to provide a novel and improved rock reinforcement system of the type wherein a resin-anchored bar is tensioned by application of excess torque after the resin is mixed and hardened, which is reliable in operation and low in cost.
It is a further object to provide a novel nut element having a frangible portion spaced outwardly from the threaded opening for use in an improved rock reinforcement system wherein the novel nut element provides the improvement.
Another object is to provide a novel method of fabricating a nut element for use in a resin-anchored, tensioned bar rock reinforcement system.
Other objects will in part be obvious and will in part appear hereinafter.